1. Field of the Invention
This invention is generally related to the field of magnetic resonance imaging (MRI) using nuclear magnetic resonance (NMR) phenomena. It is particularly directed towards method and apparatus for realizing practical adjustably sized RF coils for use in MRI procedures.
2. Related Art
Magnetic resonance imaging is now in widespread commercial usage. Magnetic resonance spectroscopic imaging (MRSI) is also now emerging from the laboratory. For purposes of this invention, the term magnetic resonance imaging (MRI) will be used as generally encompassing both areas of endeavor.
In MRI systems, selected nuclei are excited to nuclear magnetic resonance (NMR) by suitable RF signals transmitted into an object image volume under the influence of suitable magnetic fields. Resultant NMR RF responses are detected as emanating from that same volume and subsequently are processed so as to produce spatial maps of NMR nuclei populations which appear as visual images representing cross-sections through the image volume. Necessary RF signal coupling to/from the image volume is made via tuned RF coils spatially disposed about or substantially adjacent to the image volume. Sometimes a common coil structure is used for both RF signal transmission and reception. In other instances, separate dedicated RF coil structures are utilized for the transmit and receive phases, respectively of the MRI process.
To achieve a high "filling factor" when coil structures are designed so as to completely circumscribe the patient image volume (e.g., abdomen, neck, etc.), it is desirable to have the coil structure conformed as closely as possible to the patient anatomy. Consequently, so as to permit easy insertion of the proper portion of the patient anatomy, there is typically a joint area where the turns of an RF coil are separably connected via various types of RF conductive connectors. Thus, the coil structure can be selectively opened at this joint area so as to permit admission of the patient anatomy and then closed about the anatomy making proper RF electrical connections for use in a subsequent MRI procedure (after which the joint is then again opened to permit easy egress of the patient anatomy).
The need for achieving high filling factors is especially useful, for example, with relatively low magnetic field MRI (having correspondingly lower NMR frequencies) and where so-called "surface" coils of the solenoidal type are used to wrap about the appropriate portion of the patient anatomy (e.g., abdomen, neck, etc.).
At such relatively low frequencies (e.g., about 2.77 MHz), several turns are typically used so as to increase the inductance of the coil. However, if too many turns are used, the connector joint area gets overly complicated. Thus, for lower MRI field uses, neck coils might take, for example, three turns while belt coils for imaging the abdomen may typically involve two turns.
Typically, such coils might have, for example, two microhenries of inductance for operation at 2.77 MHz with about 1600 picofarads of parallel RF tuning capacitance so as to achieve resonance at this frequency. In a typical application, RF resonace tuning is achieved with only about 100 picofarads of variable capacitance range since this is the typical range of the typical varicap used for remote RF tuning. If additional varicaps are connected in parallel so as to achieve added tuning range, then the capacitance per volt of tuning control voltage quickly becomes too great for proper fine tuning control.
With such a limited predetermined range of RF tuning capacitance, the overall inductance of the coil also must be within a similarly narrow range for proper resonance tuning (e.g., after the coil is loaded by insertion of the patient anatomy). In the example just discussed, where there is perhaps only about 100 picofarads of variable capacitance out of a total tuning capacitance of 1600 picofarads, there is only about 6% variability--implying that the total loaded coil inductance also can only have a range of about 6% for proper resonant tuning.
Since human anatomy (or animal anatomy or other imaged object volumes) does not come in uniform sizes, it is often desirable to have an appropriately sized MRI RF coil for use with a given size patient anatomy. Ideally, there would be some form of coil size adjustment that would permit an operator to easily vary the size of a coil so as to fit the patient anatomy at hand.
Although the inventors do not possess access descriptive documents, it is believed that one adjustably sized RF coil structure of a "belt-type" is commercially available from an MRI supplier named FONAR. Differently sized "belts" are apparently provided with a banana-type plug connector at each end of the belt. Any of the belts apparently can be conveniently wrapped about a patient anatomy (an abdomen) and plug connected to the pigtails of a termination box (presumably having RF tuning and/or impedance matching circuits there within). However, in this circumstance, instead of adjusting the size of a relatively fixed coil structure, one is in reality simply substituting one size coil structure for a differently sized coil structure (or perhaps leaving some excess belt at the end connection points) and plug connecting that to a common RF tuning and matching circuit.
The inventors also understand (but presently possess no documentation) that a head coi supplied by Bicker MRI Inc. for its MRI system includes a section which may be completely disconnected from other portions of the coil so as to permit easy entry of the head anatomy. It is then reconnected via a suitable RF plug connectors so as to effectively reassemble the coil about the patient's head. So far as presently known, it does not appear that such plug connected section is used to change the size of the basic coil structure in this arrangement.